The torque converter used with a vehicle automatic transmission performs power transmission between an input element and an output element via a fluid. The input element is connected to an engine and the output element is connected to the automatic transmission. As the torque converter absorbs torque fluctuations, permits torque increase and transmits torque via the fluid, its transmission efficiency is lower than that of a friction-type clutch.
In order to increase torque transmission efficiency, a lockup clutch is known which directly connects the input element and output element under running conditions which do not require absorption of torque fluctuations or increase of torque. However, improvement in torque transmission efficiency via a fluid merely by performing ON/OFF control of the lockup clutch cannot be expected, and can only have a limiting effect.
JP2002-130463A published by the Japan Patent Office on May 9, 2002 discloses a device which controls the relative rotation speed of the input element and the output element to a target relative rotation speed by holding the lockup clutch between ON and OFF.
This control device divides the vehicle running region into a lockup region wherein the input element and output element are completely engaged, a converter region wherein the input element and output element are completely released, and a slip region set between these regions. In the slip region, a controller sets the target relative rotation speed of the input element and an output element according to an engine throttle opening, and controls the supply pressure to the lockup clutch based on the target relative rotation speed.